Labour has accused Government spies of casting around for a legal opinion that suited them in the Kim Dotcom case.

Prime Minister John Key confirmed yesterday that a review of the Government Communications Security Bureau (GCSB) had been ordered in the wake of revelations it spied on the German millionaire, and could lead to significant change.

The report was expected soon.

The agency also confirmed recently that it was increasing the capacity of its legal department.

Former GCSB legal adviser Hugh Wolfensohn had left the agency since the Dotcom scandal broke.

Today, Labour deputy leader Grant Robertson released documentation showing legal advice provided to the GCSB changed five times in a week.

"That suggests the GCSB was desperately casting around for a legal opinion that suited it - one that meant it could pretend the illegal spying was lawful," he said.

Robertson said it could not be interpreted as anything other than a cover-up.

The court would not release the legal documents that GCSB held, but it had given Labour affidavits which showed that between February 20 and 27 last year there were five different versions of legal advice on Operation Debut.

Robertson said GCSB knew on February 22 that Dotcom was a New Zealand resident, and therefore spying on him was illegal.

"Yet by the end of that week, magically a justification had been found," he said.

"But because we cannot get access to that advice it is impossible to know how the GCSB justified their actions."